As the dimensions of microelectronic devices continue to shrink, and device density continues to increase, the metrology requirements for process development, monitoring, and control continue to tighten accordingly. The accuracy of parameter measurements is becoming increasingly important to optimizing both device performance and chip yield. In order to obtain an accurate and robust monitoring solution, and to avoid being surpassed by advances in microelectronics fabrication, the measurement sensitivity of metrology tools must continue to improve.
For example, the need for accurately measuring the thickness and/or composition of thin films is particularly acute in the semiconductor manufacturing industry where the thickness of these thin film oxide layers on semiconductor substrates is measured. To be useful, the measurement system must be able to determine the thickness and/or composition of films with a high degree of accuracy. There also is a need to increase the resolution of metrology systems to accurately measure dimensions of features such as so-called critical dimension (“CD”), which typically refers to the minimum line width that can be fabricated for a microelectronic device. Presently, the CD of a single line feature is on the order of about 90 nm, which is difficult to measure optically. As the critical dimensions push towards the 45 nm range, there is a need for improved measurement techniques and a desire to minimize the additional cost necessary to develop such techniques.
Presently preferred measurement systems rely on non-contact, optical measurement techniques, which can be performed during a semiconductor manufacturing process without damaging the wafer sample. Such optical measurement techniques include directing a probe beam to the sample over a relatively large area and measuring one or more optical parameters of the reflected probe beam. Such a large-scale approach can be inadequate for increasingly small features, as the shape and size of such sub-micron features can be difficult to measure with such an approach.
In order to increase measurement accuracy and to gain additional information about the target sample, a number of optical measuring devices can be incorporated into a single composite optical measurement system. For example, the present assignee has marketed a product called OPTI-PROBE, which incorporates many systems, including a Beam Profile Reflectometer (BPR), a Beam Profile Ellipsometer (BPE), and a Broadband Reflective Spectrometer (BRS). Each of these devices can measure parameters of optical beams reflected by, or transmitted through, a target sample. Detailed descriptions of assignee's multiple angle of incidence devices can be found in the following U.S. Pat. Nos. 4,999,014; 5,042,951; 5,181,080; 5,412,473; 5,596,411; and 6,429,943, all of which are hereby incorporated herein by reference. The composite measurement system can combine the measured results of each of the measurement devices to precisely derive the thickness and composition of a thin film and substrate of a target sample, and/or to measure critical dimensions and feature profiles of periodic structures on samples such as semiconductor wafers. A summary of metrology devices currently found in the Opti-Probe can be found in PCT application WO/9902970, published Jan. 21, 1999. The precision of the results measured by such a system, however, still can be limited by the resolution of each of the combined systems.